1. Field of the Invention
This invention relates generally to correcting positional change (drift) of a workpiece from a nominal position, and more particularly to setting up an apparatus and using the apparatus to detect and correct substrate drift in a semiconductor processing system.
2. Description of the Related Art
A robot is commonly used to transport a substrate, such as a silicon wafer, from one location to another in semiconductor processing equipment. For example, wafers must be transported from a storage cassette and a wafer holder inside the processing chamber. The robot includes an end effector to pick up the wafer from the cassette, transfer and place the wafer into the processing chamber and then transfer the wafer back into its storage cassette after processing is complete.
The wafer must often be placed with great accuracy. A typical wafer 10 and a susceptor 12 for holding the wafer within a single-wafer processing chamber are shown in the diagram in FIG. 1. For a wafer with a diameter of 200 mm, the pocket on the susceptor, into which the wafer fits, has a diameter only slightly larger, such as 201 mm. There is a very small clearance 14, only 0.5 mm in the illustrated case, between the edge of the wafer 10 and the edge of the susceptor pocket. It is important that the wafer be centered in the pocket and not touch the sidewalls thereof. If the wafer has contact with the sidewalls of the pocket, local temperature changes occur, resulting in temperature gradients across the wafer. This can cause non-uniformity in process results, as most semiconductor processing depends critically on temperature. Similarly, uncentered wafers can be damaged during placement in a number of different handling situations.
The wafer does not normally change position with respect to the end effector during wafer transport. Errors in final placement of the wafer, known as “wafer drift,” are due mainly to variations in wafer position in the cassette at pickup, i.e., the end effector attaches to each wafer at a slightly different location. Therefore it is necessary to correct the position of the wafer before it is placed onto the wafer holder.
Often standalone stations are established for locating the center of a wafer before being picked up again by the robot, such that centered placement on the robot end effector is assured. Unfortunately, such systems require separate drop-off and pick-up operations which consume valuable processing time. It is therefore advantageous to correct the wafer's position “in line” or en route. One way this correction is made is by altering the drop-off point for the wafer transfer robot based on measurements of the wafer position after it is removed from the cassette.
In the prior art, there are many ways to measure the position of the wafer on the robot before the wafer is placed on the susceptor or other destination. It is desirable to avoid contact with the wafer, so optical systems are widely used. A light beam is shone onto a wafer, and sensors detect either a reflected beam or a portion of a transmitted beam when the robot is at a known position. Sensor data is used to determine the wafer position.
Most methods used to to correct the wafer position are based on optical through beam sensors that generate an ON/OFF switching output signal. A typical ON/OFF type optical sensor consists of a transmitter and a receiver. The transmitter generates an optical ray (which may be within the visible spectrum), which is picked up by a receiver. If the beam is blocked by an object between the transmitter and the receiver, such as a wafer, the output signal state of the sensor changes, for example from OFF to ON. Most of these sensors are made with lasers. In systems for measuring wafer position, when a wafer edge crosses the beam path, the sensor state changes and a register is triggered to record the wafer's position. Since the change in sensor state is synchronized with the recording of the wafer position, it is possible to determine the position of the wafer based on the time of wafer state change, the speed of robot movement, and the recording of the robot position. The actual wafer position is thus calculated and the subsequent placement operation uses this actual wafer position.
The accuracy of the optical measurements depends, in part, on how well the position of these optical components are known. Currently, these systems are positioned using mechanical means, which are not always accurate. Moreover, typical in line wafer centering systems are rather complex and require many sensors accurately positioned.
A need exists for a simple and reliable system for properly positioning workpieces, such as wafer, in robotic transfer.